Caption: Graphite crystal. Computer model of the molecular structure of a graphite crystal. Graphite is used in pencil leads and as a lubricant. It is composed of parallel layers of hexagonally arranged carbon atoms. Within each layer the carbon atoms (spheres) are linked by strong covalent bonds (grey rods), while the parallel layers are linked together by weak Van der Waals' forces (pink lines). This Van der Waals' bonding is strong enough to hold the layers together, yet weak enough to let them slide over each other. This also confers graphite's softness and its ability to act as a lubricant.